1994 7.3 IDI hard hot start

[medusa]

So a bit of backstory real quick will help my question make more sense. I bought a 1994 IDI f250 a few months back. Got a screamin' deal on it because it had intermittent low power and was a pretty beat up work truck. Mechanically, everything is in tip top shape however, it was a fleet truck and I have the maintenance records from the last 10+ years and was the tech maintaining it for the last 3 (I work for the company this truck came from). It just didn't have the power to pull the heavy trailers anymore and the drivers didn't know how to drive a diesel properly. Because of the damage they did, I had to replace all the injectors, the lift pump, the entire fuel return line system, retime the pump (with a tool rented from a member of this site), and run through basically all the engine performance pinpoint tests from the factory service manual.

I bought the truck knowing all this, because like I said, it was a great deal.

I expected to replace the high pressure fuel pump, sooner rather than later, and I believe that time has come, but I wanted to check and make sure I didn't miss something stupid during my diagnostics.

Here is the current symptom that makes me think the high pressure pump is done for: cold, this thing starts up like nobody's business. Better than my boss's 7.3 PSD with the block heater plugged in for 4+ hours. Not a bit of hesitation, and tons of power. After driving for a few minutes, it feels like it really looses power though. If I drive across town (10 minutes on surface roads and 10 minutes on the highway) and then park it for more than 30 minutes or so, it will not start up again. Repeat, it will not start hot after sitting for more than 20 or 30 minutes. If I am lucky, I can get it to start after waiting 2 hours or so, but it usually takes 3+ hours before I can get it to start again.

So it starts cold really well. Starts hot only if it hasn't sat fore more than 30 minutes. If it sits hot for 30 minutes to 2-3 hours, it won't start.

I'm thinking the injection pump is just worn out and when it heat soaks from sitting with no fuel running through it to cool it, the case expands faster than the pump mechanism and it can no longer build enough pressure to fire the injectors. I have verified that fuel does come out of the injector lines when hot, but it doesn't seem to spray out with much force.

I tried pouring a bucket of lukewarm water over it when hot to try and make it start, but that didn't work.

Any suggestions on anything I may have missed? Or am I ready for a new injection pump?

 

[cpdenton]

I have heard pour cold water over it. Probably a lot.

Sounds like you need to give a call to a reputable rebuilder.

Conestoga or r&d.

 

[BDCarrillo]

You replaced the injectors and now the tired old IP has to fight them. It's typically advised to replace the IP and injectors at the same time.

The Stanadyne DB2 rebuild manual strongly recommends against thermally shocking the IP.

 

[medusa]

You replaced the injectors and now the tired old IP has to fight them. It's typically advised to replace the IP and injectors at the same time.

The Stanadyne DB2 rebuild manual strongly recommends against thermally shocking the IP.

I forgot to mention that the injection pump had been replaced with a locally rebuilt unit about 4 years ago. My concern is that it was a poor rebuild, and needs replacement again due to a combination of wear, abuse, and the rebuild just not being high quality.

And more to the point, I'm concerned that I may have missed something during my diagnosis and I'm asking the opinion and help of others who can approach this with a different thought pattern than my own. I am prepared to replace the IP. I just don't want to if I don't have to.

 

[BDCarrillo]

Can you elaborate on the hot start issue by posting what you've tried? Does it go through several cranking cycles without even trying to light? Do the glow plugs cycle normally? Does it light and run for 5-10 seconds, then die?

 

[medusa]

Can you elaborate on the hot start issue by posting what you've tried? Does it go through several cranking cycles without even trying to light? Do the glow plugs cycle normally? Does it light and run for 5-10 seconds, then die?

Glow plugs will light and run very briefly on first key cycle. If I turn off the key without cranking, and then turn back to the run position, the glow plugs will run normally. Several 10-15 second cranking cycles will eventually make the engine sound like it is trying to start, but letting off the key will cause it to die without hesitation. Sometimes, I am able to get it to start if I pump the accelerator while it is cranking.

 

[BDCarrillo]

I infrequently have the same problem, but I've narrowed it down to air intrusion. A quick press of the schrader valve on the filter head after a 10-15 second crank will let you know. It's also a long shot, but does it act up if run and shut down on both tanks? Any luck on hot starts with ether (3 minutes after cycling glow plugs, leaving key in run)?

I am running straight WMO so my similar symptoms may be from a different cause.

 

[medusa]

I infrequently have the same problem, but I've narrowed it down to air intrusion. A quick press of the schrader valve on the filter head after a 10-15 second crank will let you know. It's also a long shot, but does it act up if run and shut down on both tanks? Any luck on hot starts with ether (3 minutes after cycling glow plugs, leaving key in run)?

I am running straight WMO so my similar symptoms may be from a different cause.

I've performed air intrusion tests multiple times and have no air intrusion issues. The schrader valve always spits fuel after cranking, no air. Same symptom regardless of which tank I'm on. I've never left the key on for 3 minutes and then attempted to start it, I'll have to try that.

 

[BDCarrillo]

Key on for 3 minutes is only to prevent untimed detonation from hot glow plugs when using ether (starting fluid). If it starts on ether and chugs/lugs for 10-20 seconds it's very likely an air intrusion issue in/around the return lines OR fuel delivery from the lift pump. Engine heat may cause a plastic cap to slightly expand, allowing air to enter and fuel to drain back to the tank.

Once it cools and contracts, the residual air is trapped and forced to the filter head. Perhaps a thorough check of the return lines/caps for dampness is in order. It's very easy to damage an o-ring on install.

 

[medusa]

Key on for 3 minutes is only to prevent untimed detonation from hot glow plugs when using ether (starting fluid). If it starts on ether and chugs/lugs for 10-20 seconds it's very likely an air intrusion issue in/around the return lines OR fuel delivery from the lift pump. Engine heat may cause a plastic cap to slightly expand, allowing air to enter and fuel to drain back to the tank.

Once it cools and contracts, the residual air is trapped and forced to the filter head. Perhaps a thorough check of the return lines/caps for dampness is in order. It's very easy to damage an o-ring on install.

Wouldn't this also cause a hard cold start upon next startup? I'll definitely check that either way.

 

[BDCarrillo]

It depends on where the air bubble ends up. I've observed similar issues on warm re-starts with known leaks on the original return caps (150k miles, new parts in the mail). Installing a new lift pump only seemed to make it worse for me.

Once again, I run WMO so my similar issues may be more related to the viscosity of my fuel.

On a different line of thinking, might the heat soak be affecting the cranking amp output of your batteries or the efficiency of your starter?

 

[medusa]

It depends on where the air bubble ends up. I've observed similar issues on warm re-starts with known leaks on the return caps. Installing a new lift pump only seemed to make it worse for me.

Once again, I run WMO so my issues may be more related to the viscosity of my fuel.

On a different line of thinking, might the heat soak be affecting the cranking amp output of your batteries or the efficiency of your starter?

With a tach attached for diagnostic purposes, the cranking speed doesn't seem to be affected by heat. Maybe a 10 or 15 rpm difference, but nothing out of the ordinary.

 

[BDCarrillo]

Well scratch that then. Back to the heat soaked IP idea. Since the final injection pressure is the result of steel rollers in an aluminum housing, it's not unreasonable to suspect the IP.

 

[medusa]

Well scratch that then. Back to the heat soaked IP idea. Since the final injection pressure is the result of steel rollers in an aluminum housing, it's not unreasonable to suspect the IP.

So double check for air leaks, especially from fuel return caps, and if I can't find air leaks then it's most likely the IP?

 

[BDCarrillo]

I'm not an expert, but that would seem reasonable based on the information given and my similar issues with my known problems.

Research across other forums does point to the IP as the likely culprit.